System and Method for Detecting False Sharing

ABSTRACT

In one embodiment, a method of false sharing detection includes performing, by a device, a plurality of optimization passes on source code, to produce optimized source code and receiving, by the device, selection criteria, The method also includes adding instrumentation to the optimized source code, by the device, after performing the plurality of optimization passes, to produce an instrumented code, where the instrumentation is configured to track memory access addresses and access types of global variables and heap variables in accordance with the selection criteria.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/335,621, filed on Jul. 18, 2014, which claims the benefit of U.S.Provisional Application Ser. No. 61/858,824 filed on Jul. 26, 2013, andentitled “System and Method for Detecting False Sharing,” whichapplication is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a system and method for multithreading,and, in particular, to a system and method for detecting false sharing.

BACKGROUND

Multithreading on multiple cores is often used in computing. Multiplecores are used in a variety of devices, including smart phones, tablets,laptops, workstations, supercomputers, and data centers. Multithreadingis a programming and execution model which utilizes the underlyinghardware resources by running different threads on different hardwarecores concurrently. These threads may share data, files, andinput/output (I/O) in order to facilitate cooperatively completing aspecified task.

One challenge in multithreading is false sharing, which is related tocache usage. Cache, which is accessed much faster than main memory, isused by central processing units (CPUs) to accelerate programexecutions. Before accesses, the CPU checks whether the data to beaccessed is in the cache. When the data is already stored in the cache,the CPU directly accesses the data from the cache, reducing accesslatency by avoiding accessing the slower main memory. When the data isnot already stored in the cache, the CPU automatically fetches the datato the cache from the main memory in blocks of a fixed size, referred toas cache lines.

In an example multicore system, the cores have their own private caches.Thus, data accessed by threads running on different cores may beduplicated in caches of those involved cores. A cache coherence protocolis invoked to facilitate correct accesses from different threadsconcurrently. When the data of a cache line in one core has beenchanged, the cache coherence protocol invalidates other copies of thesame cache line in other cores so changes made by one core are seen bythe other cores.

This cache line level coherency creates a false sharing problem. Whenthreads running on different cores access different locations in thesame cache line, every write by one core on the cache line invalidatesthe cache line copies on the other core. As a result, frequent cacheline invalidation may degrade performance, because other cores withtheir cache entries invalidated have to re-fetch the data from the mainmemory, using CPU time and memory bandwidth. Also, false sharing mayfurther degrade performance when a system has more cores or a largercache line size.

SUMMARY

In an embodiment method of false sharing detection, the method includesperforming, by a device, a plurality of optimization passes on sourcecode, to produce optimized source code and receiving, by the device,selection criteria. The method also includes adding instrumentation tothe optimized source code, by the device, after performing the pluralityof optimization passes, to produce an instrumented code, where theinstrumentation is configured to track memory access addresses andaccess types of global variables and heap variables in accordance withthe selection criteria.

An embodiment device includes a non-transitory memory storage includinginstructions and one or more processors in communication with thememory. The one or more processors execute the instructions to perform aplurality of optimization passes on source code to produce optimizedsource coude and receive selection criteria. The instructions alsoinclude instructions to add instrumentation to the optimized source codeafter performing the plurality of optimization passes, to produce aninstrumented code, where the instrumentation is configured to trackmemory access addresses and access types of global variables and heapvariables in accordance with the selection criteria.

In an embodiment computer program product for installation on a device,the computer program product includes programming for execution by thedevice. The programming includes instructions for performing a pluralityof optimization passes on source code to produce optimized source codeand receiving selection criteria. The programming also includesinstructions for adding instrumentation to the optimized source codeafter performing the plurality of optimization passes, to produce aninstrumented code, where the instrumentation is configured to trackmemory access addresses and access types of global variables and heapvariables in accordance with the selection criteria.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates an embodiment multicore system;

FIG. 2 illustrates a graph of runtime versus the number of threads in amulticore system;

FIG. 3 illustrates a cache line;

FIG. 4 illustrates false sharing in a cache line;

FIG. 5 illustrates true sharing in a cache line;

FIGS. 6A-B illustrate cache invalidation in multiple cores;

FIG. 7 illustrates interleaved access leading to performance problems;

FIG. 8 illustrates tracking of cache lines;

FIG. 9 illustrates an embodiment system for false sharing detection;

FIG. 10 illustrates another embodiment system for false sharingdetection;

FIG. 11 illustrates a flowchart for an embodiment method of falsesharing detection;

FIG. 12 illustrates a flowchart for an embodiment method of insertinginstrumentation into source code;

FIG. 13 illustrates a flowchart for an embodiment method of trackinginvalidations;

FIGS. 14A-F illustrate invalidation tracking;

FIG. 15 illustrates a flowchart for an embodiment method of thresholdbased invalidation tracking;

FIG. 16 illustrates embodiment pseudocode for invalidation tracking;

FIG. 17 illustrates a graph of performance overhead;

FIG. 18 illustrates a graph of memory usage overhead; and

FIG. 19 illustrates a block diagram of an embodiment general-purposecomputer system.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

In an embodiment a predictive software-based false sharing detector isused. An embodiment tracks accesses within virtual cache lines, whichare contiguous memory ranges spanning actual hardware cache lines, topredict false sharing on hardware platforms. A runtime system iscombined with compiler instrumentation to track cache invalidations. Thecompiler instruments memory accesses so the runtime system is notifiedwhen an access is executed. The runtime system collects and analyzesactual memory accesses to detect and report false sharing. A user mayadjust instrumentation granularity based on performance requirements.There may be a tradeoff between performance and precision of detection.False positives may be located with compiler based instrumentation. Byteand word changes are differentiated at the compiler. An embodiment isindependent of thread libraries. For example, it may be applied toportable operating system interface (POSIX) threads (pthreads), messagepassing interface (MPI), open multi-processing (OpenMP), and otherthreading libraries. A threshold invoked tracking mechanism may be usedto reduce the memory overhead. An embodiment algorithm captures cacheinvalidations based on read and write accesses of a cache line.

Multithreading in multicore systems increases the processing speed. FIG.1 illustrates multicore system 100 containing three cores, core 102,core 106, and core 110. The cores may communicate with each other inperforming computations. Separate tasks are performed on the differentcores. The cores have their own cache. For example, core 102 has cache104, core 106 has cache 108, and core no has cache 112. Three cores arepictured, but fewer or more cores may be used (e.g., 2, 4, 8, etc.).When a core accesses data, it examines its cache to determine whetherthe data is already stored in its cache. When the data is already in thecache, the core reads the data from the cache. However, when the data isnot already in the cache, the core downloads the data from main memoryto the cache, and then accesses the data.

It is desirable to have more threads to reduce the runtime. However,because of multithreading issues, such as false sharing, the runtimedoes not increase rapidly. FIG. 2 illustrates a graph of actual runtime120 and expected runtime 122 based on the number of threads. Falsesharing slows the program by 13 times with 8 threads.

False sharing may occur due to code. For example, software which leadsto false sharing is:

int count[8] int W; void increments(int S) {  for(in=S; in<S+W; in++)  for(j=o; j<1M; j++)    count[in]++; } int main(int THREADS) {W=8/THREADS; for(i=o; i<8; i+=W)  spawn(increment,i);

FIG. 3 illustrates cache line 130 with cache words 132, 134, 136, and138. In one example, a cache line has 64 bytes and 8 words of 8 bytes.False sharing occurs when different threads update or access differentwords in the same cache line. FIG. 4 illustrates an example of falsesharing. Task 1 accesses word 142 in cache line 140, task 2 accessesword 144 in cache line 140, task 3 accesses word 146 in cache line 140,and task 4 accesses word 148 in cache line. In true sharing, differentthreads update or access the same word in the same cache line. FIG. 5illustrates an example of true sharing. Task 1 and task 2 both accessword 154 in cache line 150, which also includes words 152, 156, and 158.False share degrades performance, while true sharing does not. Forexample, false sharing slows MySQL™ by 50%.

FIGS. 6A-B illustrate an example false sharing with two threads. Task174 runs on core 172 and task 186 runs on core 184. Core 172 has its owncache, with cache line 176 having words 178, 180, 182, and 185.Likewise, core 184 has its own cache, with cache line 188 having words190, 192, 194, and 196. Cache may be located in the same chip as theprocessor, and has higher access speeds than the main memory, such asmain memory 162. When data is fetched to the cache from the main memory,it is fetched in units of cache lines. For example, a cache line may be32 bytes, 64 bytes, 128 bytes or another number of bytes. The use ofcache lines may reduce the number of fetches.

However, cache lines may cause performance problems when there is falsesharing. Different cache coherence protocols handle updates differently.Examples of cache protocols used for cache invalidation include modifiedshared invalid (MSI) protocol and modified exclusive shared invalid(MESI) protocol. In the MSI protocol, the cache lines may have threepossible states: modified, shared, or invalid. In the modified state,the block has been modified in the cache, and the data in the cache isinconsistent with that in the main memory. A cache line with a modifiedstate has the responsibility to write the block to the main memory whenit is evicted. In a shared state, the cache line is unmodified andexists in at least one cache. The cache in the shared state may evictthe data without writing it to the main memory. In the invalid state,the cache line is invalid, and should be fetched from memory or anothercache. In the MESI protocol, cache lines may be modified, exclusive,shared, or invalid. When a cache line is exclusive, the cache line ispresent only in the current cache, but it matches the main memory. Inone example, the state is changed to shared in response to a readrequest. Alternatively, it is changed to modified when written to. Inboth protocols, when a cache line is written to, it goes to theinvalidate state. For example, task 174 writes to word 178 in cache line176. Core 184 has a copy of the same data in its cache. The data isinvalidated to ensure correct data for the case of true sharing.However, invalidation is unnecessary for false sharing.

When task 2 access data in cache line 188, for example in FIG. 6B, task187 fetches the data from core 172. Likewise, when task 186 modifiesword 206 in cache line 188, the data in core 172 is invalidated, and isread from the main memory. When there are many interleaved writes fromdifferent threads, there may be many cache invalidations. The largenumber of cache invalidations may cause serious performance problems. Asthe number of cores increases or the cache line size increases, falsesharing becomes more problematic.

False sharing occurs in a variety of situations. For example, falsesharing may occur on struct fields, two different global variables, twodifferent heap objects, two different fields of the same structure, ortwo different elements of the same array. Some situations which may leadto false sharing include:

me = 1; you = 1; // globals me = new Foo; you − new Bar; // heap class X{  int me;  int you; }; / / fields array[me] = 12; array[you] = 13; //array indicesFalse positives occur in runtime, and are not visible from the sourcecode.

False sharing may causes performance problems. FIG. 7 illustratesflowchart 210 indicating how false sharing causes performance problems.In step 212, interleaved access causes cache invalidations 214. Thesecache invalidations cause performance problems 216.

In one example, physical addresses for the false sharing are reported,which involves a large overhead. In another example, a false sharingmiss ratio is reported, which also involves a large overhead. In anadditional example, the cache miss ratio and cache invalidation ratioare reported, which does not pinpoint the exact cause of the falsesharing. In another example, a binary instrumentation tool is used,which does not access the source code, but introduces a high performanceoverhead, because every instruction is intercepted and interpretedonline. Also, this approach does not report detailed information aboutfalse sharing on the binary level. Alternatively, a binaryinstrumentation technique intercepts every memory read and write access,which also has a high overhead of about 100 times. In another example,Valgrind is used to track the sequence of memory accesses and report theworst case estimation of false sharing, which has an extremely highoverhead of about 200 times.

In a simulation approach, a simulation simulates the execution of aprogram and finds cache misses. The simulation approach may be slow, andalso may rely on the correctness of the simulation tool. The exacthardware arguments may be used.

In an example, a performance tuning utility (PTU) points out functions,but may not pinpoint objects with false sharing problems. PTU usesspecialized hardware, for example precise event based sampling (PEBS)registers. PTU only runs on special hardware, and does not extend toother hardware, such as computers or mobile devices using ARMarchitecture. Also, PTU may report many false positives caused by truesharing, heap object re-usage, and non-interleaved false sharing.

In another example, Sheriff uses a processes-as-threads framework.Sheriff tracks memory modification of different threads by using memoryprotection and a twinning-and-diffing mechanism. Sheriff turns threadsinto processes to utilize the memory protection mechanism of multipleprocesses to capture writes from different threads. To determine whichfalse sharing instances actually cause performance problems, Sheriffcaptures interleaved writes from different threads and ranks theseriousness of the false sharing problems based on the rating. Sheriffdetects write-write false sharing for applications using pthreadswithout self-defined synchronizations. Also, Sheriff may not perform adhoc synchronization using a stack for communication.

An embodiment method of detecting false sharing leverages the compilerto selectively instrument memory accesses by inserting callbackfunctions provided by a runtime library. The callback functions assistthe runtime system in capturing or collecting the actual memoryread/write information for an application. Based on the runtimeinformation, whether false sharing poses a problem is detected. Bothwrite-write false staring and read-write false sharing may be detected.Also an embodiment may detect scalability problems in software caused byfalse sharing.

To detect false sharing, cache lines with many cache invalidations maybe detected. FIG. 8 contains memory 222, which may contain globalvariables, heaps, etc. Status words 224 track cache lines in memory 222.When memory is accessed from a thread, information related cacheinvalidations are updated. Objects involved in cache lines with a largenumber of cache invalidations are updated.

An embodiment is referred to as predator. System 300 illustrated by FIG.9 includes compiler instrumentation 302 and runtime system 304. Thecompiler instrumentation is inserted into the source code to trackmemory read and write accesses. When there is a memory access, theruntime system is notified, for example using a library call. Theruntime system collects memory accesses, updates correspondinginvalidations, and reports false sharing. The callsites of the memoryallocations are saved. Graphing memory access helps separate falsesharing from true sharing.

A compiler may capture instructions accessing global variables and heapobjects. Other information, such as when those instruments are beingexecuted or how many times a variable or pointer is accessed, is notdetermined during the compiling phase, because this information dependson the input parameter or execution environment. Such dynamicinformation is used to detect false sharing. The runtime system captureswhen instructions are executed and how many times a variable or apointer is accessed. An embodiment combines a runtime system andcompiler instrumentation to provide detailed information to detect falsesharing in applications.

In an embodiment, a compiler selectively instruments read and writeaccesses of global variables and pointers. Instrumentation refers tousing the compiler to insert function callbacks into the source code,for example when the application invokes read and write accesses onglobal variables or heap variables. Callback functions facilitate theruntime system collecting memory read and write information. Thus, anembodiment detects false sharing problems.

FIG. 10 illustrates flowchart 320 for a method of detecting falsesharing. Flowchart 320 includes compiling phase 322 and runtime phase324. Compiling phase 322 includes source code 326, selectiveinstrumentation 328, and linking library 330, while runtime phase 324includes runtime system 332. Source code 326 is the source code wherefalse sharing is to be avoided. Selective instrumentation 328instruments source code 326 in a flexible fashion, so read and writeaccesses to the memory are provided to the runtime system. Selectiveinstrumentation reduces performance overhead. This flexibility may befacilitated by making different decisions based on the access type,sampling target, and sampling optimization at different levels. Then,instrumentation is linked to a library in linking to library 330.

In one example, only write accesses are instrumented to detectwrite-write false sharing. Alternatively, both read and write accessesare instrumented to detect read-write false sharing problems as well aswrite-write false sharing problems. In read-write false sharing, onethread is writing to a cache line while other threads are reading fromthe same cache line.

Different sampling targets may be chosen, for example based on userinput. In one example, all functions inside all modules are sampled.This leads to the runtime system obtaining all memory read and writeinformation about the variables and objects at the expense of moreperformance overhead. In another example, a user provides a black listfor some modules, functions, or variables not to be instrumented. Thecompiler skips instrumentation on the black listed items, which reducesthe performance overhead from instrumentation. In another example, auser provides a red list for modules, functions, or valuables to beinstrumented. The compiler selectively instruments the red listed itemsto reduce the performance overhead from the instrumentation. Differentsampling targets may be selected to provide reasonable performanceoverhead. There is a tradeoff between performance and accuracy.

Sampling optimization may be performed on different levels. At the basicblock level, sampling is selected once for multiple accesses to the sameaddress. The compiler informs the runtime system how many accesseshappen in the first basic block. Thus, the correct number of memoryaccesses in one basic block may be obtained if there is no flow switchinside the basic block. The sampling overhead may be thus reduced.

Because the compiler is leveraged for instrumentation, very fine grainedinformation about every access may be obtained. For example, whether theaccess is to a specific word, byte, or bit may be determined. Thisinformation may be used to precisely locate false sharing in thereporting phase.

Runtime system 332 detects false sharing. Cache line invalidation is aroot cause for performance degradation, because cache invalidationswaste both CPU time and memory bandwidth. Therefore, an embodimentsearches for the memory accesses which may introduce a large amount ofcache line invalidation.

When a thread writes to a cache line immediately after other threadshave accessed the same cache line, the write operation is likely tocause at least one cache invalidation. An embodiment data structure andmethod detects cache invalidations caused by interleaved access.

Instrumentation provides memory access information to the runtimesystem, which detects false sharing based on the sequence of memoryaccesses on the cache lines. The performance overhead of a specificprogram is proportional to the degree of instrumentation. Moreinstrumentation leads to more performance overview.

In one embodiment, instrumentation is added once per type of memoryaccess on addresses to the same basic block. This selectiveinstrumentation may not affect the effectiveness of detection. Lesstracking of accesses inside a basic block may induce fewer cacheinvalidations without impacting the overall behavior of cacheinvalidations.

Instrumentation may be dynamic instrumentation or compilerimplementations. Dynamic instrumentation approaches may analyze theprogram's code before the execution to insert instrumentation. Thisintroduces significant performance overhead, for example caused byrun-time encoding and decoding, but provides good generality, becauserecompilation is not used. Compiler instrumentation insertsinstrumentation during the compilation phase, which may have lessgenerality.

FIG. 11 illustrates flowchart 370 for a method of detecting falsesharing. Initially, in step 372, instrumentation is performed in thecompiler phase. An embodiment compiler uses low level virtual machine(LLVM) to perform instrumentation at the intermediate representationlevel. The compiler traverses functions one by one, searching for memoryaccesses, for example to global and heap variables. For memory accesses,a function is instrumented to invoke the runtime system with the memoryaccess address and access type, read access or write access. In oneexample, accesses to stack variables is omitted, because stack variablesmay be used for local storage. Alternatively, accesses to stackvariables are instrumented. In an example, the instrumentation is placedat the end of the LLVM optimization passes, so only memory accessessurviving previous LLVM optimization passes are instrumented.

Then, in step 374, cache invalidations are tracked. This is performed bythe runtime system. The runtime system collects memory accesses byhandling those function calls inserted during the compilerinstrumentation phase. The cache invalidations are analyzed to determinewhether they constitute false sharing.

Finally, in step 376, false sharing is reported. For global variablesinvolved in false sharing, the name, address, and size are reported. Forheap objects, the callsite stack for their allocations, their address,and size are reported. Also, the word granularity access information forcache lines involved in false sharing, including which threads accessedwhich words, may be shared. This information may assist in diagnosingand fixing false sharing.

FIG. 12 illustrates flowchart 410 for a method of applyinginstrumentation to the code during compilation. Initially, in step 412optimization passes are performed on source code. For example, LLVMoptimization is performed. In one example, the optimization passes areperformed before the instrumentation is applied to the source code. Someor all of the optimization passes may be performed before theinstrumentation is applied.

In step 414, selection criteria are received. For example, selectioncriteria may be received from a user. The selection criteria may includespecific items to be instrumented. Alternatively, the selection criteriainclude specific items not to be instrumented. In another example, theselection criteria indicate that all items should be instrumented. Theamount of instrumentation may be adjusted based on the user'srequirements.

Finally, in step 416, instrumentation is inserted into the source code.The instrumentation is inserted to track cache line accesses. In oneexample, both read accesses and write accesses are tracked.Alternatively, only write accesses are tracked.

An embodiment data structure used to track cache invalidations is a twoentry cache status table which tracks accesses for the cache lines.There may be one table per cache line. This table maintains the accesshistory for the cache lines. The entries contain a thread identificationnumber (ID) and an access type (read access or write access). The fieldsare used to update the table with new access.

Global variables or heap objects on cache lines with a large number ofcache invalidations may be reported. FIG. 13 illustrates flowchart 340for a method of updating a table for tracking accesses for a cache line.This may be performed for each cache line. Initially, in step 364, anaccess is performed. The access is either a read access or a writeaccess. The access time is determined.

Next, in step 342, the system determines whether the access is a readaccess. When the access is a read access, the system proceeds to step344. When the access is a write access, the system proceeds to step 352.

In step 344, the system determines whether the table is full. When thetable is full, the system proceeds to step 348, where it does not recordthe new access. When the table is not full, the system proceeds to step346.

The system determines whether the existing entry has a different threadID from the current access in step 346. The system compares the threadID of the entry to the thread ID of the access. When the existing entryhas a different thread ID from the current access, the system proceedsto step 350, and records the access. A new entry is recorded to thetable. When the existing entry has the same thread ID as the currentaccess, the system proceeds to step 348 and does not record the newaccess.

Also, in step 352, the system determines whether the table is full. Whenthe table is full, the system proceeds to step 360, and when the tableis not full, the system proceeds to step 354.

In step 354, the system determines whether the existing entry has adifferent thread ID from the thread ID of the current access. When thethread IDs are the same, the system replaces the entry in the table withthe current access in step 356. In this case, there is no invalidation.When the existing thread ID in the table is different from the thread IDof the access, the system proceeds to step 358. In step 358, the systemcleans up the table, writes the new access, and records an invalidation.

In step 360, the system determines whether the existing entry has adifferent thread ID from the thread ID of the access. When the threadIDs are the same, the system proceeds to step 362. In step 362, thesystem cleans up the table and records the write access. However, thereis no invalidation. When the thread IDs are different, there is aninvalidation, and the system proceeds to step 358, where the table iscleaned up, a write access is recorded, and an invalidation is recorded.

After the number of cache invalidations for the cache lines isdetermined, the seriousness of false sharing for the cache lines isranked. Cache lines with more cache invalidations are more likely tohave a false sharing problem, which may degrade performance.

FIGS. 14A-F illustrate the tracking of invalidations for two threads,thread 1 (T1), and thread 2 (T2). The threads run on separate cores,each with a corresponding separate private cache. Infinite cachecapacity is assumed. The hardware cache is mimicked at the softwarelevel. FIG. 14A illustrates two entries in a table, entry 232 and entry234. Initially, these entries are both set to zero. The number ofinvalidations is also tracked in entry 236. The number of invalidationsis initially set to zero. A time sequence of read and write accesses tomemory is in sequence 238.

Shown by FIG. 14B, thread 2 reads from the cache line. Thread 2 is thensaved in entry 232, indicating that thread 2 has accessed the cacheline. Entry 234 and entry 236 remain zero.

Next, illustrated by FIG. 14C, thread 2 writes to the cache line. Afterthread 2 writes to the cache line, thread 1 reads from the cache line.Entry 234 is set to T1, indicating that T2 has accessed the cache line.Entry 236, the number of invalidations, is still zero.

In FIG. 14D, thread 2 then writes to the cache line. The table iscleared, an invalidation is recorded, and the write access is recorded.Entry 234 is set to 0, and the number of invalidations, entry 236, isset to 1.

Then, thread 1 writes to the cache line in FIG. 14E. Entry 232 is set toT1, the number of invalidations is set to 2, and entry 234 remains zero.

Next, thread 1 reads from the cache line, and there is no change toentries 232, 234, and 236. Finally, as illustrated by FIG. 14F, thread 2writes to the cache line. The number of invalidations is increased to 3.Also, entry 232 is set to T2. Entry 234 remains 0.

In one example, a threshold for the number of write accesses is used todetermine whether there is a high risk of false sharing. When the numberof write accesses on a cache line is above a predefined threshold, theread and write accesses are tracked for each word in the cache line.Thread read or write accesses on a word, and the number of totalaccesses, may be tracked. This information may differentiate truesharing from false sharing. Also, the location of the problem may bedetermined. Using a threshold may reduce the overhead by only trackingdetails when there is an increased risk of false sharing.

After cache lines with a large number of cache invalidations aredetected, actual false sharing is differentiated from true sharing. Intrue sharing, multiple threads update the same counter in the cachelines, which cause a large number of cache invalidations.

The access information for the words in cache lines involved in falsesharing is tracked. The number of read and/or write accesses to thewords by is tracked by thread. When a word is accessed by multiplethreads, the origin of this word is marked as a shared access. The wordis marked as do not track for further accesses. This informationfacilitates distinguishing false sharing from true sharing in thereporting phase. Also, the information helps diagnose where actual falsesharing occurs when there are multiple fields or multiple objects in thesame cache line. This may reduce the effort to fix false sharingproblems.

To report the origins of heap objects with false sharing problems,callsite information for heap objects is maintained. Source code levelinformation for the heap objects may be reported. To obtain callsiteinformation, memory allocations and de-allocations are intercepted. Forexample, the backtrace( ) function in the glibc library is used toobtain the whole callsite stack. False positives may be avoided byupdating recording information at memory de-allocations for objectswithout false sharing problems. Heap objects involved in false sharingare not reused.

For accesses, the corresponding cache line's metadata is looked up toobtain detailed information or to update access counters. In oneexample, a shadow memory mechanism is used to store metadata for piecesof application data. Thus, corresponding metadata is directly computedand located based on address arithmetic.

To support shadow memory, a predefined starting address and fixed sizefor a heap may be used. A custom memory allocator is used, which may usea per-thread-heap mechanism. In the allocator, memory allocations fromdifferent threads do not occupy the same physical cache line, whichautomatically avoids false sharing among different objects.

In one embodiment, a threshold based tracking mechanism is used. Becausecache invalidations are the root cause of performance degradation, andonly write accesses introduce cache invalidations, cache lines with asmall number of writes are unlikely to be a significant performancebottleneck. In one example, cache invalidations are tracked once thenumber of write accesses to a cache line crosses a pre-definedthreshold, known as a tracking threshold. Before the threshold isreached, only the number of write accesses on a cache line is tracked,while read accesses are not tracked.

FIG. 15 illustrates flowchart 390 for a threshold based trackingmechanism. In step 392, the system determines whether the number ofwrite accesses is greater than or equal to the tracking threshold. Whenthe number of write accesses is greater than or equal to the trackingthreshold, the system proceeds to step 396 to track detailedinformation. However, when the number of writes is less than thetracking threshold, the system continues to track only the number ofwrite accesses in step 394.

In one example, two arrays on shadow memory are maintained. CacheWritestracks the number of memory writes on cache lines, and CacheTrackingtracks detailed information on cache lines when the number of writes ona cache line exceeds the tracking threshold. When the threshold is notreached, CacheTracking is not used. Example pseudocode 380 isillustrated in FIG. 16.

When the number of write accesses on a cache line is greater than thetracking threshold, accesses are tracked to store details such as wordaccess information, update access counter, and the cache access historytable for the cache line. When a cache line is involved in false or truesharing, updating those counters exacerbates the impact of false sharingon performance. Not only is there an invalidation on an applicationcache line, but there is also at least one other cache invalidationcaused by updating the metadata of the corresponding cache lines. Toreduce performance overhead, an embodiment only samples the firstspecified number of accesses of a sampling interval for tracked cachelines. In one example, there is an access counter for each cache line,but only the first 10,000 accesses of every million accesses on a cacheline is tracked, for a 1% sampling rate. Different sampling rates do notnegatively impact effectiveness.

An embodiment may provide suggestions for fixing false sharing problemsbased on the memory trace information is provided. This may reduce themanual overhead of fixing false sharing problems.

Different inputs may cause different executions of a program. When aspecific input does not exercise a portion of the code with falsesharing problems, that false sharing problem is not detected. However,inputs may be generalized over to find latent false sharing problems onthose exercised codes. When a reasonably representative set of inputsare exercised, false sharing may be effectively detected.

Input size may affect detection results. An embodiment introducesseveral threshold values to reduce the tracking overhead, which may beadjusted based on actual detection environments. When the input size issufficiently small that it cannot generate enough false sharing eventsto pass the pre-defined thresholds, the detection mechanism may not betriggered. However, a larger input size may trigger the mechanism.

Memory hierarchy of the underlying machine does not affect the detectionresults. An embodiment does not attempt to obtain the actual cacheinvalidations of a program, which may depend on real memory hierarchy.Thus, an embodiment does not bind to a specific machine, providing goodgenerality.

FIG. 17 illustrates a graph of the overhead performance of predatorusing normalized runtime. Bars 310 shows the baseline, bars 312 showspredator non-prediction (NP), and bars 314 shows predator. Predator hasa performance overhead of about six times. For 16 benchmarks, predatorimposes a 5.4 times performance overhead. Five programs, histogram,kmeans, bodytrack, ferret, and swaptions have more than an 8 timesperformance overhead. The histogram benchmark runs more than 26 timesslower than the original executions with pthreads library, becausetracking detailed access on cache lines with false sharing exacerbatesfalse sharing. For bodytrack and ferret, although there is no falsesharing, predator detects a large amount of cache lines with writeslarger than tracking threshold. Thus, tracking those accessing detailsfor those cache lines imposes performance overhead. Predator imposes asmall performance overhead for input-output (IO)-bound applications suchas matrix multiply, blackscholes, ×264, aget, Memcached, pbzib2, andpfscan, because predator does not add any performance overhead for IOoperations.

FIG. 18 illustrates the memory usage using normalized memory usage. Bars400 shows the baseline and bars 402 shows the memory usage for predator.Predator imposes less than 50% memory overhead for 17 out of 22applications. For swaptions and aget, predator introduces more memoryoverhead, because the original memory footprints are very small at only3 kilobytes. Adding the code of detection, prediction, and reportingcontributes to a large ratio of memory overhead. Although the averagememory usage of applications is over two times, the total memory usageoverhead is only about 40% on predator.

FIG. 19 illustrates a block diagram of processing system 270 that may beused for implementing the devices and methods disclosed herein. Specificdevices may utilize all of the components shown, or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The processing system may comprise a processing unitequipped with one or more input devices, such as a microphone, mouse,touchscreen, keypad, keyboard, and the like. Also, processing system 270may be equipped with one or more output devices, such as a speaker, aprinter, a display, and the like. The processing unit may includecentral processing unit (CPU) 274, memory 276, mass storage device 278,video adapter 280, and I/O interface 288 connected to a bus.

The bus may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. CPU 274 may comprise any type of electronic dataprocessor. Memory 276 may comprise any type of non-transitory systemmemory such as static random access memory (SRAM), dynamic random accessmemory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), acombination thereof, or the like. In an embodiment, the memory mayinclude ROM for use at boot-up, and DRAM for program and data storagefor use while executing programs.

Mass storage device 278 may comprise any type of non-transitory storagedevice configured to store data, programs, and other information and tomake the data, programs, and other information accessible via the bus.Mass storage device 278 may comprise, for example, one or more of asolid state drive, hard disk drive, a magnetic disk drive, an opticaldisk drive, or the like.

Video adaptor 280 and I/O interface 288 provide interfaces to coupleexternal input and output devices to the processing unit. Asillustrated, examples of input and output devices include the displaycoupled to the video adapter and the mouse/keyboard/printer coupled tothe I/O interface. Other devices may be coupled to the processing unit,and additional or fewer interface cards may be utilized. For example, aserial interface card (not pictured) may be used to provide a serialinterface for a printer.

The processing unit also includes one or more network interface 284,which may comprise wired links, such as an Ethernet cable or the like,and/or wireless links to access nodes or different networks. Networkinterface 284 allows the processing unit to communicate with remoteunits via the networks. For example, the network interface may providewireless communication via one or more transmitters/transmit antennasand one or more receivers/receive antennas. In an embodiment, theprocessing unit is coupled to a local-area network or a wide-areanetwork for data processing and communications with remote devices, suchas other processing units, the Internet, remote storage facilities, orthe like.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method of false sharing detection, the methodcomprising: performing, by a device, a plurality of optimization passeson source code, to produce optimized source code; receiving, by thedevice, selection criteria; and adding instrumentation to the optimizedsource code, by the device, after performing the plurality ofoptimization passes, to produce an instrumented code, wherein theinstrumentation is configured to track memory access addresses andaccess types of global variables and heap variables in accordance withthe selection criteria.
 2. The method of claim 1, wherein adding theinstrumentation into the optimized source code comprises insertinginstrumentation for read accesses and for write accesses.
 3. The methodof claim 1, wherein adding the instrumentation into the optimized sourcecode comprises inserting instrumentation for write accesses and notinserting instrumentation for read accesses.
 4. The method of claim 1,wherein the selection criteria comprises a black list of items not to beinstrumented.
 5. The method of claim 1, wherein the selection criteriacomprises a red list of items to be instrumented.
 6. The method of claim1, wherein the selection criteria indicates that all items beinstrumented.
 7. The method of claim 1, wherein receiving the selectioncriteria comprises receiving the selection criteria from a user.
 8. Adevice comprising: a non-transitory memory storage comprisinginstructions; and one or more processors in communication with thememory, wherein the one or more processors execute the instructions to:perform a plurality of optimization passes on source code, to produceoptimized source code; receive selection criteria; and addinstrumentation to the optimized source code after performing theplurality of optimization passes, to produce an instrumented code,wherein the instrumentation is configured to track memory accessaddresses and access types of global variables and heap variables inaccordance with the selection criteria.
 9. The device of claim 8,wherein the instructions to add the instrumentation into the optimizedsource code comprise instructions to insert instrumentation for readaccesses and for write accesses.
 10. The device of claim 8, wherein theinstructions to add the instrumentation into the optimized source codecomprise instructions to insert instrumentation for write accesses andnot inserting instrumentation for read accesses.
 11. The device of claim8, wherein the selection criteria comprises a black list of items not tobe instrumented.
 12. The device of claim 8, wherein the selectioncriteria comprises a red list of items to be instrumented.
 13. Thedevice of claim 8, wherein the selection criteria indicates that allitems be instrumented.
 14. The device of claim 8, wherein theinstructions to receive the selection criteria comprise instructions toreceive the selection criteria from a user.
 15. A computer programproduct for installation on a device, the computer program productcomprising programming for execution by the device, the programmingincluding instructions for: performing a plurality of optimizationpasses on source code, to produce optimized source code; receivingselection criteria; and adding instrumentation to the optimized sourcecode after performing the plurality of optimization passes, to producean instrumented code, wherein the instrumentation is configured to trackmemory access addresses and access types of global variables and heapvariables in accordance with the selection criteria.
 16. The computerprogram product of claim 15, wherein adding the instrumentation into theoptimized source code comprises inserting instrumentation for readaccesses and for write accesses.
 17. The computer program product ofclaim 15, wherein adding the instrumentation into the optimized sourcecode comprises inserting instrumentation for write accesses and notinserting instrumentation for read accesses.
 18. The computer programproduct of claim 15, wherein the selection criteria comprises a blacklist of items not to be instrumented.
 19. The computer program productof claim 15, wherein the selection criteria comprises a red list ofitems to be instrumented.
 20. The computer program product of claim 15,wherein the selection criteria indicates that all items be instrumented.